[gicV3] GICv3 virtualization support, to allow zircon-guest to run on QEMU

PD-50 #Done

Change-Id: I60a6937ad9a0515d1d737fbe562be517eb9a66ef

[kernel][gic] GIC code refactoring to add support for GICv3 in Hypervisor

Change-Id: Ia3e96db67ca507a26fbf537d8eb004c1d5a9b6b0

[arm64][hypervisor] Start basic VCPU execution.

While this works on QEMU, it fails with an EC of 0b000000 on the
HiKey960. It looks like the IPA isn't being calculated correctly, so
I've got to continue investigating that in a follow up CL.

ZX-1241 #comment In Progress

Change-Id: I3a97978b8fa3fc65166936ed2f02344edc30c427

[arm64][hypervisor] Add El2CpuState.

Add code to manage EL2 CPU state. Currently this does the very minimum
and sets the EL2 stack for each CPU.

Next, I'll start setting up more of the EL2 state and also guest
physical address space.

Change-Id: I18b7f9d00b236e52cdc317dffe3b42fcffbcb8fe

[arm64] Check boot EL2 when creating a guest.

This mirrors the VMX check in the x86, and helps us to early exit when
creating a hypervisor guest if it's not supported on the device.

Also split the hypervisor.cpp file into guest.cpp and vcpu.cpp. It's
still a skeleton, but it helps while we flesh it out.

Change-Id: I482f55b5a448c6b26acb363ec3bad76a9ed8c5dc

[hypervisor] Invert build dependencies.

This allows us to depend on lib/magenta from lib/hypervisor, so we can remove
all of the WITH_LIB_MAGENTA defines and still build cleanly with the -test
targets.

Change-Id: I7a54c2ae4d812213a21ff67b963626a65879b2ea